Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
FIG. 1 is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN). The E-UTRAN may include one or more evolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from eNodeB 20 to UE 10, and “uplink” refers to communication from the UE to an eNodeB. UE 10 refers to communication equipment carried by a user and may be also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNodeB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for UE 10. The eNodeB and MME/SAE gateway may be connected via an S1 interface.
The eNodeB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNodeB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 20.
The MME provides various functions including distribution of paging messages to eNodeBs 20, security control, idle state mobility control, SAE bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling. The SAE gateway host provides assorted functions including termination of U-plane packets for paging reasons, and switching of the U-plane to support UE mobility. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway”, but it is understood that this entity includes both an MME and an SAE gateway.
A plurality of nodes may be connected between eNodeB 20 and gateway 30 via the S1 interface. The eNodeBs 20 may be connected to each other via an X2 interface and neighboring eNodeBs may have a meshed network structure that has the X2 interface.
FIG. 2(a) is a block diagram depicting architecture of a typical E-UTRAN. As illustrated, eNodeB 20 may perform functions of selection for gateway 30, routing toward the gateway during a Radio Resource Control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of Broadcast Channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the E-UTRAN, and as noted above, gateway 30 may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, supporting a Packet Data Convergence Protocol (PDCP) function, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of Non-Access Stratum (NAS) signaling.
FIGS. 2(b) and 2(c) are block diagrams depicting the user-plane protocol and the control-plane protocol stack for the E-UTRAN. As illustrated, the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well-known in the art of communication systems.
The physical layer, the first layer (L1), provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. Between different physical layers, namely, between physical layers of a transmission side and a reception side, data is transferred via the physical channel.
The MAC layer of Layer 2 (L2) provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 (L2) supports the transmission of data with reliability. It should be noted that the RLC layer illustrated in FIGS. 2(b) and 2(c) is depicted in dashed lines because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself is not required. The PDCP layer of Layer 2 (L2) performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). Here, the RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the UTRAN.
As illustrated in FIG. 2(b), the RLC and MAC layers (terminated in an eNodeB 20 on the network side) may perform functions such as Scheduling, Automatic Repeat Request (ARQ), and Hybrid Automatic Repeat Request (HARQ). The PDCP layer (terminated in eNodeB 20 on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.
As illustrated in FIG. 2(c), the RLC and MAC layers (terminated in an eNodeB 20 on the network side) perform the same functions as for the user plane. As illustrated, the RRC layer (terminated in an eNodeB 20 on the network side) may perform functions such as broadcasting, paging, RRC connection management, Radio Bearer (RB) control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway 30 on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE 10.
The NAS may be divided into three different states; first, a LTE_DETACHED state if there is no RRC entity in the NAS; second, a LTE_IDLE state if there is no RRC connection while storing minimal UE information; and third, an LTE_ACTIVE state if the RRC connection is established. Also, the RRC may be divided into two different states such as a RRC_IDLE and a RRC_CONNECTED.
In RRC_IDLE state, the UE 10 may receive broadcasts of system information and paging information while the UE specifies a Discontinuous Reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area. Also, in RRC-IDLE state, no RRC context is stored in the eNodeB.
In RRC_CONNECTED state, the UE 10 has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the network (eNodeB) becomes possible. Also, the UE 10 can report channel quality information and feedback information to the eNodeB.
In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 10 belongs. Therefore, the network can transmit and/or receive data to/from UE 10, the network can control mobility (handover) of the UE, and the network can perform cell measurements for a neighboring cell.
In RRC_IDLE mode, the UE 10 specifies the paging DRX (Discontinuous Reception) cycle. Specifically, the UE 10 monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle.
The paging occasion is a time interval during which a paging signal is transmitted. The UE 10 has its own paging occasion.
A paging message is transmitted over all cells belonging to the same tracking area. If the UE 10 moves from one tracking area to another tracking area, the UE will send a tracking area update message to the network to update its location.
The procedure whereby the UE 10 sends a first message to the network is referred to as initial access. For initial access, the common uplink channel called RACH (Random Access Channel) is used. In all cases in GSM and UMTS systems, the initial access begins with the UE 10 sending a connection request message including a reason for the request and the network responding with an indication of radio resources allocation for the requested reason.
To send information over the air, a physical random access procedure is used. The physical random access transmission is under control of a higher layer protocol that performs important functions related to priority and load control. This procedure differs between GSM and UMTS radio systems. As the present innovation is UMTS enhancement/evolution related, the W-CDMA random access procedure is described.
The transport channel RACH and two physical channels PRACH and AICH, are involved in the random access procedure. The transport channels are the channels supplied from the physical layer to the protocol layer (MAC). There are several types of transport channels to transmit data with different properties and transmission formats over the physical layer.
Physical channels are identified by code and frequency in FDD mode and are normally based on a layer configuration of radio frames and timeslots. The form of radio frames and timeslots depends on the symbol rate of the physical channel.
The radio frame is the minimum unit in the decoding process, consisting of 15 time slots. A time slot is the minimum unit in the Layer 1 bit sequence. Thus the number of bits that can be accommodated in one time slot depends on the physical channel.
The transport channel RACH (Random Access Channel) is an uplink common channel used for transmitting control information and user data. It is applied in random access and used for low-rate data transmissions from the higher layer.
RACH is mapped to the uplink physical channel called PRACH (Physical Random Access Channel). AICH (Acquisition Indication Channel) is a downlink common channel, which exists as a pair with PRACH used for random access control.
A random access channel is considered a contention based uplink transmission based on transmission of a random-access burst. The random access burst includes a random access preamble portion and a message payload portion. Due to simultaneous access of several users, collisions may occur such that the network cannot decode the initial access message. The UE 10 can start the random-access transmission (both preambles and message) at the beginning of an access slot only.
The random access preamble portion is used, for example, for signature detection and uplink synchronization. The message payload portion includes any data or control signaling information.
The time axis of both the RACH and the AICH is divided into time intervals, called access slots. There are 15 access slots per two frames, with one frame having a length of 10 ms or 38400 chips. The access slots are spaced 1.33 ms (5120 chips) apart. Information regarding which access slots are available for random-access transmission and which timing offsets to use between RACH and AICH, between two successive preambles and between the last preamble and the message is signaled by the network.
The preamble is a short signal that is sent before the transmission of the RACH message. A preamble consists of 4096 chips, which is a sequence of 256 repetitions of Hadamard codes of length 16 and scrambling codes assigned from the upper layer. The Hadamard codes are referred to as signature of the preamble. There are 16 different signatures and a signature is randomly selected from available signature sets on the basis of ASC and repeated 256 times for each transmission of preamble portion.
The message portion is spread by OVSF codes that are uniquely defined by the preamble signature and the spreading codes as the ones used for the preamble signature. The message portion radio frame of length 10 ms is divided into 15 slots, each consisting of 2560 chips.
Each slot consists of a data portion and a control portion that transmits control information, such as pilot bits and TFCI. The data portion and the control portion are transmitted in parallel.
The 20-ms-long message portion consists of two consecutive message portion radio frames. The data portion consists of 10*2 k bits (k=0, 1, 2, 3), which corresponds to the Spreading Factor of 256, 128, 64 or 32.
The AICH consists of a repeated sequence of 15 consecutive access slots, each of length 40 bit intervals or 5120 chips. Each access slot consists of two portions, an Acquisition Indicator (AI) portion consisting of 32 real-valued signals a0, . . . , a31 and a portion of duration 1024 chips where transmission is switched off.
FIG. 3 is a block diagram illustrating a RACH burst structure when OFDM modulation is used. The receiver receives the RACH burst used by OFDM modulation and performs a correlation between the received signal and the available preamble signatures. Depending on the output of the correlator, a specific algorithm determines whether or not a RACH burst has been sent.
If the detector determines that a signature has been sent with a high probability, the timing advance value is estimated and the received signature is determined. The receiver can then perform a channel estimation based on the timing advance and the signature and demodulate the data symbols.